The present invention relates to a method for the preparation of silica glass or, more particularly, relates to an efficient and inexpensive method for the preparation of silica glass by the so-called sol-gel method in which a silica sol solution containing silica particles is concentrated to have the silica particles settled to form a structure of closest packing followed by elevation of the temperature to convert the sol into a gel, drying the silica gel and sintering and vitrification of the dried silica gel into silica glass.
Synthetic silica glass of high purity is a promising material having applications in the optical industry and optocommunication and the demand therefor is rapidly increasing in recent years. Several methods are known in the prior art for the preparation of silica glass including the method of pyrolytic or high-temperature synthesis, in which a silicon compound having vaporizability is subjected to flame hydrolysis in an oxyhydrogen flame or to oxidative decomposition in a plasma flame to deposit silica particles on a target followed by fusion and vitrification of the thus deposited silica particles, and the method of low-temperature synthesis or the so-called sol-gel method, in which a solution of an alkoxy silane in an alcohol or an aqueous solution of an inorganic silicate such as water glass is converted into a gel and the thus formed silica gel is heat-treated and vitrified into transparent silica glass.
These known methods of prior art have several problems and disadvantages. For example, the former method of high-temperature synthesis is disadvantages in respect of the large energy consumption in the form of the heat of combustion of the oxyhydrogen flames or in the form of an electric power for the generation of the plasma as well as the heat required in the step of vitrification of the deposited silica particles at a temperature of about 2000.degree. C. and the low take-up of the dopant added to the silica deposit and low controllability of the dopant concentration which is essential for the control of the refractive index of the silica glass due to the vaporization loss of the dopant in the course of the high temperature treatment for sintering and vitrification of the silica particles as well as the lengthy procedure of the process leading to a great economical disadvantage.
The latter method of low temperature synthesis is classified into three types. In the first type (A) of the method, an alkoxy silane compound is first hydrolyzed in an alcoholic solution in the presence of an acidic catalyst such as hydrochloric acid or a basic catalyst such as ammonia followed by heating to convert the reaction mixture into a wet gel which is then dried, sintered and vitrified into transparent silica glass (see, for example, Japanese Patent Publication No. 59-9497). In the second type (B) of the method disclosed in Journal of American Ceramic Society, volume 66, No. 10, page 683 (1983), a suspension of very finely divided silica particles having a particle diameter of 1 .mu.m or smaller in a suitable dispersion medium such as water is converted into a wet gel by adding an acidic or basic gelation catalyst followed by drying, sintering and vitrification into transparent silica glass or, alternatively, the dispersion medium of the above mentioned dispersion of the fine silica particles is evaporated to dryness to form a dry agglomerate of the silica particles which is then sintered and vitrified into transparent silica glass. In the third type (C) of the method of low temperature synthesis, an alkoxy silane compound is hydrolyzed in the presence of an acidic or basic catalyst to form a uniform sol which is admixed with a separately prepared suspension of fine silica particles having a diameter of 1 .mu.m or smaller in a dispersion medium such as water or, alternatively, the sol of the hydrolysis product is admixed with the fine silica particles as such to prepare a silica sol solution containing the silica particles dispersed therein followed by control of the pH and temperature to cause gelation thereof into a wet gel which is dried, sintered and vitrified into transparent silica glass (see, for example, Japanese Patent Kokai No. 61-91033).
The above described methods of low temperature synthesis are advantageous over the method of high temperature synthesis from the standpoint of energy saving since the vitrification can be performed at a temperature rarely exceeding 1000.degree. to 1500.degree. C. In addition, various kinds of dopants can be introduced into the silica glass by the cohydrolysis of a mixture of an alkoxide of the dopant element and an alkoxy silane compound or by adding a powdery oxide of the dopant element to the alkoxy silane compound subjected to hydrolysis so that the dopant take-up can be almost 100% without loss thereof by vaporization. Further, the step of gelation of the sol into a gel can be performed in a vessel of any desired form so that the silica glass after vitrification may also have a desired form and the costs for the subsequent working of shaping, cutting, grinding and the like can greatly be saved to give an economical advantage.
The methods of low temperature synthesis, however, also cannot be free from several problems. For example, the method of the first type is not suitable for manufacturing a large block of silica glass since the mass of the gel is liable to be cracked or to cause foaming in the course of drying of the gel and subsequent sintering. The method of the second type is also not suitable for manufacturing a large block of silica glass since the mass of the gel is liable to be cracked in the course of drying although it is free from the problem of foaming in addition to the economical disadvantage due to the relatively high temperature of 1500.degree. C. or higher required for the vitrification of the dried gel. The method of the third type is defective as a method for obtaining transparent silica glass with good reproducibility and high yield because the mass of the gel under vitrification is sometimes subject to foaming resulting in products of opaque silica glass.